JP2008160130A - Optoelectronic device including a diffraction grating with a lateral structure - Google Patents

Abstract

Novel functionality is provided by a new type of grating that is not only preferentially integrated into a BRS structure, but also suitable for any other type of stripe structure, to improve the performance of optoelectronic devices. obtain.
An optoelectronic device has a stripe structure including at least one embedded waveguide and an elongated stripe-shaped layer including a mechanism, a layer called a lattice layer, each mechanism having a substantially rectangular shape, The length is approximately perpendicular to the length of the stripe of the grating layer, the layer is arranged to provide optical coupling with the light wave propagating in the waveguide, and the length of the particular mechanism is greater than the width of the waveguide The stripe structure is substantially small.
[Selection] Figure 4

Description

  The field of the invention is that of optoelectronic devices. The present invention is applicable and suitable for many structures. The invention applies in particular to so-called BRS structures.

  Many optoelectronic devices have been made with various technologies, including a technology called BRS (acronym for Buried Ridge Stripe), which is known as an embedded stripe. Means incorporating a gain-coupled or index-coupled diffraction grating along the single-mode waveguide. In particular, the names of a laser called DFB (distributed feedback) laser, a distributed Bragg grating element called DBR (distributed Bragg reflector), an optical amplifier called SOA (semiconductor optical amplifier), and an optical filter may be mentioned.

  This type of device is generally made by a method that includes the three main rough steps shown at 1, 2 and 3, which are shown in FIG. In general, because of the size of these elements, many of these are made simultaneously. Thus, FIG. 1 shows the fabrication of three elements. The figure shows the element in top and cross-sectional views, where the cross-section is a cut plane conventionally indicated by a vertical dotted line and two horizontal straight arrows.

  In the first step, the layer 2 constituting the future single-mode optical waveguide is produced on the substrate 1. This first layer is then covered with a second layer 3, which constitutes a Bragg grating 4 once etched.

  In the second step, the combination of the two layers 2 and 3 is engraved or etched to define the width of the waveguide and the Bragg grating covering it. This step is also illustrated in FIG. 2, which is a perspective view of a portion of the device at this stage of fabrication. In this figure, the grid 4 has a “top hat” profile.

  In a third or final step, an epitaxial regrowth of a suitable material 5 is performed to embed the waveguide. Thus, a light guide and electrical injection can be provided.

  This method has major drawbacks. This is because, as can be seen in FIG. 1, the zone with the grating must be wide enough to allow re-alignment of the waveguide by photolithography. This tolerance is estimated to be about 1 micron. This technique therefore does not allow any change in the diffraction grating along the horizontal axis of the waveguide. By tampering with either the change in layer thickness defined during the epitaxy of the structure, or the change in the aperture ratio of the grating, defined when the grating is engraved or etched, the pitch of the grating in the longitudinal direction is adjusted. It can only be adjusted.

  Now, the interaction between the optical mode and the grating determines the essential optical properties of the device, such as spectral selectivity. More precisely, the spectral characteristics depend on the position and shape of this grating relative to the waveguide. Furthermore, with this method of the prior art, the properties and optical characteristics of the resulting device are necessarily limited.

  One possible solution to remedy the above disadvantages is to create a “ridge” structure as shown in FIG. In this case, the grating 4 is etched laterally at right angles to the average plane of the waveguide, not on the waveguide. In this way it is possible to achieve a certain degree of control over the lateral shape of the grating. However, these particular structures have been used to make diode lasers, but also have a number of drawbacks. This is because, as can be seen from FIG. 3, it is necessary to etch the layer deeply and the etching thickness is about 10 times that of the conventional BRS structure. Of course, the coupling with the optical mode propagating through the waveguide is only lateral, resulting in a low modulation of the coupling coefficient. Furthermore, it is technically impossible to etch the grating across the width of the stripe in the laser section. Finally, electrical injection is disadvantageous with high series resistance and low radiated power.

  The object of the present invention is to provide new functionality through a new type of grating that not only improves performance or is preferentially integrated into the BRS structure, but is also suitable for any other type of stripe structure. Is to get. In general, these novel gratings have a pitch that can vary along at least one axis perpendicular to the direction of the waveguide. A fabrication method for fabricating these new structures is proposed.

  More precisely, the subject of the present invention is an optoelectronic device comprising at least one buried waveguide in the form of an elongated stripe and a layer, also referred to as a grating layer, also comprising a feature, each feature being An optoelectronic device having a generally rectangular shape, wherein the length of the mechanism is approximately perpendicular to the length of the stripe of the grating layer, and wherein the layer is arranged to provide optical coupling with a light wave propagating in the waveguide The length of the specific mechanism is substantially smaller than the width of the waveguide.

  Advantageously, the mechanism is a recess made in or around the lattice layer, so that the layer has an indentation.

  Advantageously, the mechanism is of variable size and is arranged symmetrically or asymmetrically with respect to the central axis of the grating layer.

  Advantageously, the lattice layer comprises at least one row of features, the axis of the row being parallel to the central axis of the lattice layer, or the lattice layer comprises at least two or three rows of features. The axis of the row may be parallel to the central axis of the lattice layer.

  Advantageously, all rows of features are identical in shape.

  Advantageously, the pitch separating two consecutive features is constant in each row, or the pitch separating two successive features in the first row is two consecutive in the second row. It is different from the pitch separating mechanism.

The present invention also relates to a method for making a device comprising a substrate according to one of the aforementioned claims, which method comprises at least the following three steps. That is,
The deposition of the waveguide layer on the substrate, the deposition of the grating layer, and the subsequent etching of the grating;
Continuation of the waveguide etching down to the protection of the grating region and to the substrate,
Epitaxial regrowth to embed waveguides and gratings;
It is.

  The present invention will be understood more clearly and other advantages will become apparent upon reading the following description, presented as a non-limiting example, and thanks to the accompanying drawings.

  FIG. 4 shows a perspective view of a BRS structure according to the present invention with the upper layer removed to reveal the diffraction grating 4 for clarity.

  The lattice layer has an elongated stripe shape including the mechanisms 41, and each mechanism 41 has a substantially rectangular shape. The length of the mechanism is approximately perpendicular to the length direction of the stripe of the lattice layer. The length LM of a particular feature is substantially smaller than the width LR of the waveguide layer stripe.

  Starting from this basic arrangement, there are many possible configurations. As a non-limiting example, many configurations are shown in FIGS. For simplicity and to clarify the changes in structure, these figures show only the grating 4 in the top view. For clarity, the mechanism 41 is always shown as a rectangular shape. Of course, the methods used to make these mechanisms can result in substantially different shapes, such as oblong or rounded shapes.

  FIG. 5 shows a grating 4 in which the mechanism 41 is a recess made around the grating layer and thus this layer has an indentation. As shown in FIG. 5, these indentations may vary in length. In the case of FIG. 5, the length of the notch varies linearly, but other variations are possible. Thus, a fine adjustment of the coupling coefficient is possible for coupling between the grating and the waveguide along the waveguide.

  In FIG. 6, the mechanism 41 is a recess formed inside the lattice layer. Here again, it becomes possible to adjust the coupling coefficient for coupling between the grating and the waveguide along the waveguide by tampering with the length of the mechanism.

  The gratings in FIGS. 5 and 6 allow fine tuning of the coupling coefficient, but are particularly suitable for DFB lasers. This is because the longitudinal distribution of light intensity in the laser is strongly dependent on the product K × L, where K is the coupling coefficient and L is the length of the laser. A high K × L leads to a large concentration of photons in the center of the cavity. In contrast, a low K × L results in photons concentrating at the end of the cavity. Now, single mode radiation can only be maintained if a uniform photon density distribution is established in the cavity. It is possible to produce this type of structure by making a grating with a variable coupling coefficient.

  FIG. 7 shows a lattice in which the rectangular features 41 are recesses that differ in shape along both the longitudinal and lateral directions of the stripe, and the pitch separating the various features is also variable. In this way, a very complex adjustment of the coupling coefficient can be achieved.

  FIG. 8 shows a grating 4 in which the recesses 41 have a variable pitch P and the pitch P1 of the mechanism on the right side of the figure is smaller than the pitch P2 on the left side of FIG. This arrangement makes it possible to produce DBR grating elements that reflect at two different wavelengths.

  FIG. 9 shows a lattice in which the mechanism 41 is concentrated at the end of the element. In this way it is possible to obtain a very high reflection coefficient over a short section located at the end of the element. This type of grating may be used in particular in an SOA amplifier. Thus, the wave passing through the SOA will reciprocate many times in the waveguide coupled to the grating, greatly increasing its amplification.

FIG. 10 shows a grid 4 having a row of features 41 that are all identical and centered on the central axis of the grid. The length of the mechanism is smaller than the width of the grid. The advantage of this type of grating is that it provides the possibility to generate different couplings depending on the order of the modes propagating in the waveguide. The notch in the center of the waveguide favors coupling in the fundamental mode, resulting in a more restrictive width for single mode operation. The reason is that the fundamental mode energy is greatest at the center of the waveguide, as shown in FIG. 10 which shows the change in luminous intensity I of the fundamental mode M ₀ plotted in bold as a function of the waveguide width LR. Because. Conversely, as also shown in FIG. 10, the energy of the primary mode M ₁ is minimum at the center of the grating and maximum at the edges. This mode is therefore not preferred by the grid. If the device is a DFB laser, the cut-off width of the single mode waveguide is considerably increased. Thus, the maximum radiated power of the laser is increased. Of course, it may be possible to prioritize a mode other than the basic mode. All that is required is to place the mechanism at the energy maxima of the mode. Thus, the notch along the waveguide edge favors coupling in the second order mode.

  FIG. 11 shows a set of two gratings 4 and 8 made in the same grating layer. The first lattice 4 is composed of a row of mechanisms 41 that are all the same and concentrate on the central axis of the lattice layer. This first grating mechanism has a first pitch. The second grating 8 consists of two rows of mechanisms 81 that are all identical and are symmetrically positioned around the grating layer. The mechanism of the second grating has a second pitch that is different from the first pitch. Therefore, a two-mode waveguide may be fabricated. The first grating promotes coupling in the fundamental mode of the first wave, while the second grating facilitates coupling in the first mode of the second wave.

  FIG. 12 shows a set of two gratings 4 and 8 made in the same grating layer and separated by a central groove 6. If the gratings thus produced have different pitches, it is possible to guide two different optical modes in parallel in each grating, each mode being guided by one of the gratings. .

  FIG. 13 shows a set of two gratings 4 and 8 made in the same grating layer. Each grid consists of the same series of features, but the pitch of the two grids is different. With this type of grating, it is possible to produce lasers that emit at two different wavelengths. It would be possible to generate the same function using a single row of mechanisms. In this case, the column will have two sections: a first section with a first pitch and a second section with a second pitch that is different from the first pitch.

  Of course, there are many possible alternative embodiments of the lattice, similar to the above embodiment. Those skilled in the art will know how to adapt these configurations according to the desired characteristics of the final device without any particular difficulty.

  A method of fabricating a device according to the present invention with a BRS structure is illustrated in FIG. This method includes three essential steps.

  In the first step, a waveguide layer 2 is deposited on the substrate 1, followed by a grating layer 3, and then the waveguide / grating assembly is etched to create a grating recess. You may use the electron beam lithography technique also called electron beam etching. These techniques make it possible to achieve very fine etching with a precision of a few nanometers and allow great freedom in the geometric design of the mechanism.

  In the second step, a photoresist is used to protect only the lattice zones. The waveguide etching is then continued until it reaches the substrate.

  In the third step, conventional epitaxial regrowth of a suitable material 5 is performed to embed the waveguide. Thus, a light guide and electrical injection can be provided.

3 shows three general steps in the fabrication of a BRS structure according to the prior art. 2 shows a perspective view of a prior art BRS structure at the end of the second step shown in FIG. FIG. 6 shows a perspective view of a ridge structure according to an alternative embodiment of the prior art. The perspective view in the manufacturing method of the BRS structure by this invention is shown. Fig. 2 shows a top view of a grating according to the invention in various possible configurations. 3 shows three general steps in the fabrication of a BRS structure according to the present invention.

Explanation of symbols

1 substrate 2 first layer (waveguide layer)
3 Second layer (lattice layer)
4 Diffraction grating (first grating)
5 Material 8 Diffraction grating (second grating)
41 Mechanism 81 Mechanism LM Mechanism Length LR Waveguide Layer Stripe Width

Claims (11)

  An optoelectronic device comprising at least one embedded waveguide in the form of an elongated stripe and a similarly elongated stripe-shaped layer comprising a feature, called a grating layer, each feature having a substantially rectangular shape, the length of the feature Is an optoelectronic device arranged to provide optical coupling with a light wave propagating through the waveguide, wherein the layer is substantially perpendicular to a length direction of the stripe of the lattice layer, An optoelectronic device characterized in that its length is substantially smaller than the width of the waveguide, the grating layer comprises at least two rows of features, and the axis of the rows is parallel to the central axis of the grating layer.   The optoelectronic device according to claim 1, wherein the mechanism is a recess formed in the lattice layer.   The optoelectronic device according to claim 1, wherein the mechanism is a recess made around the lattice layer, and thus the layer has an indentation.   The optoelectronic device according to claim 1, wherein the mechanism is a variable size.   The optoelectronic device according to claim 1, wherein the mechanism is arranged symmetrically with respect to the central axis of the lattice layer.   The optoelectronic device according to claim 1, wherein the mechanism is disposed asymmetrically with respect to the central axis of the lattice layer.   The optoelectronic device according to claim 1, wherein the lattice layer comprises at least three rows of features, the axis of the rows being parallel to the central axis of the lattice layer.   The optoelectronic device according to any one of claims 1 to 7, characterized in that all the rows of mechanisms have the same shape.   9. An optoelectronic device according to claim 8, characterized in that the pitch separating two successive features is constant in each row.   9. The optoelectronic device of claim 8, wherein the pitch separating two consecutive features in the first row is different from the pitch separating two successive features in the second row. A method for producing a device comprising a substrate according to any one of claims 1 to 10 with BRS technology, comprising at least the following three steps:
-Deposition of the waveguide layer on the substrate, deposition of the grating layer, and subsequent etching of the grating;
Protection of only the grating region, and continuation of the etching of the waveguide down to the substrate;
-Epitaxial regrowth to embed the waveguide and the grating;
A method comprising the steps of:

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